Rather than looking for a new winger, it's better for Manchester United to sign a new goalkeeper and few defenders. Their midfielder and striker already strong enough, so they just wasting their money to sign a new winger.
A player will join a big and tough team, Manchester United only a big team, so Mitoma should be in Manchester City.
You're right though, United could wait until the summer window to get Mitoma instead of signing him in the winter windows cause what really need currently is a good defense line, it would help them from conceding much and support the goalie as well, i don't think Manchester United need another keeper currently, Onana might not be in a good form, but that doesn't mean he's a bad goalie, if you watch him at Intermilan you'll definitely know he's not a bad a people think all he needs currently is be surrounded with a good defense line and also to work on his confidence level.
Their defense line was really affected by injury crisis and that's one of the reasons they're performing poorly cause they lack good opinions in that area and the coach is forced to play Amrabat in a position where he's not very familiar with, therefore strengthening has defense would be more important than getting another attacker.